The secondary treatment of slag from garbage incinerator processes is already known. The journal Umweltmagazine [Environment Magazine], Nov., 1985, pp. 30-32, describes a process used to minimize the quantity of slag produced. In this process, a so-called wet deslagger, within which the slag is cooled, is arranged beneath the furnace, i.e., at the end of the furnace grate. It is also noted that the content of heavy metals is also reduced by this wet deslagging. However, this reduction of the heavy-metal content is unsatisfactory, and considerable quantities of injurious substances remain in the slag. If it remains in the wet deslagger, the slag breaks down, and this causes additional difficulties during handling. The use of a wet deslagger has been known in other plants for a considerable time, wherein, in many instances, a water-filled slag channel is used. The cooled slag is moved to a slag bunker from this water-filled channel, where it is stored until finally dumped. Very high storage and transportation weights and, as a consequence of these, very high disposal costs result from the high water content in the slag.
Very large amounts of slag are generated because of the large quantities of garbage and waste that arrive in a garbage incinerators. This slag is usually disposed of in dumps. In a few instances, slag from garbage incinerators is also used in road construction or to build noise-abatement barriers. Approximately 30% of the weight of the garbage that arrives at an incinerator ends up as slag at the end of the incineration process. The dump or the use of this slag as road-construction material is in no way entirely free of risk, for considerable quantities of soluble and harmful substances remain in the slag. The slag can contain various quantities of these harmful substances, depending on the composition of the garbage that has been incinerated. On average, the content of harmful substances in slag produced from incinerators that burn domestic garbage can very in the range from 50-100 g per kg of slag. The harmful substances contained in the slag can include, in particular, chloride, lead, cadmium, zinc, iron, copper, and other impurities. Corresponding analyses have been published, for example, in Schweizer Ingenieur und Architekt [Swiss Engineer and Architect], Vol. 19, 1983. When slag from garbage incinerators is stored in dumps, leakage water has to be collected and disposed of, otherwise it constitutes a considerable environmental hazard. Similar problems arise when such slag is used for road construction, for which reason, in many instances the slag is mixed with cement and rendered impermeable to water. Since fewer and fewer dumps suitable for slag are available, the storage or dumping of such slag is causing major problems. The use of such slag in road construction is also being restricted to a small number of applications because of the formerly high contents of harmful substances.
The publication Stuttgarter Berichte zur Abfallwirtschaft [Stuttgart Reports on Waste Management], Berlin, 1984, pp. 221-246, asserts that the major part of water-soluble harmful substances is concentrated in fly dust, and it is recommended that only dust-free slag be used, for example, in road construction. DE 37 00 608 describes a process and an apparatus for processing slag, in which, after the slag has been quenched in a water bath, the fine components with a grain size of up to 1 mm are removed. The remaining slag has a reduced content of harmful substances and can be used for highway and road construction. The known processes for cleaning the slag still do not satisfy the increasing demands made on them. The slag that has been cleaned still contains residual components of water-soluble harmful substances. The chloride content can still amount to 50% of the original quantity, for example. The fact that the slag has to be stored after having been quenched in a water bath is also a disadvantage, for large storage capacities are still required. The harmful substances in the dust components are floated in the slag by the water bath and adhere to the pores after drying. The proposed drying to a residual moisture content of 1% requires considerable amounts of energy when large quantities of slag are involved. When dried to such an extent, the cleaned slag is not suitable for use as construction material and has to be rewetted in order to be used, for example, for highway and road construction.